A torque converter interposed between a vehicle engine and an automatic transmission transmits torque between a pump impeller and a turbine runner via a fluid. The relative rotation of the pump impeller to the turbine runner increases engine fuel consumption, so after the vehicle has started, the pump impeller and turbine runner are preferably connected together via a lockup clutch as soon as possible.
In a torque converter provided with a lockup clutch, a change-over between a converter mode which transmits torque via the fluid and a lockup mode which transmits torque via the lockup clutch is performed via a slip mode which permits a slip of the lockup clutch so that part of the torque is transmitted via the fluid, and the remainder of the torque is transmitted via the lockup clutch.